Tox creates a sandboxed “virtual environment” (“virtualenv”) for each Python version, 2.7, 3.6, and 3.7.
These are stored in the .tox/ directory.
It then runs the test suite under all versions of Python, per the tox.ini file.
If tox is unable to find a Python executable on your system for one of the supported versions, it will fail for that environment.
You can safely ignore these failures when developing locally.

You can run the test suite in a single enviroment with the -e flag:

$ tox -e py37

To access the executables installed in one or the other virtual environment,
source the virtualenv into your current shell, e.g.:

$ source .tox/py37/bin/activate

You should then have, e.g., the custodian command available:

(py37)$ custodian -h

You’ll also be able to invoke nosetests or pytest directly with the arguments of your
choosing, e.g.: